The present invention relates to a screw pump of the type comprising a screw rotatably arranged in a housing the threads of which screw are partially in engagement with a rotatable sealing device comprising radial sealing discs, whereby the screw area of the screw, including thread grooves and thread walls, are formed by a generatrix which is a part of a circle arc, the circle of which is rotatable around an eccentric rotation axis when the generatrix rotates around the axis of the screw and is displaced along this with a velocity being proportional to the rotational velocity of the screw and wherein the sealing device comprises at least one circular disc the eccentric rotational axis of which, which is arranged transverse to the axis of the screw coincides with the rotation axis of the generatrix, i.e., the rotation axis of the disc is a tangent to the generatrix.
The object of the present invention is to obtain a screw pump of the above type which is very compact and which allows being introduced into so called xe2x80x9cbutterworthxe2x80x9d spaces in tankers, i.e., such having a diameter of 300 mm, but having an improved pumping capacity.
Another object is to obtain a screw pump of the said type by mews of which pumping can be carried out from a very close distance from e.g., a tank bottom, such as at the cleansing of oil tanks onboard ships, or collecting sites for spill oil at harbours.
Industrial rest products, such as chemical waste, spill oil crude oil and others, quite often have a very high viscosity, in particular at ambient temperatures of 0-25xc2x0 C. or lower as well as they are heavily polluting with wearing particles of varying sizes, in particular after discharge in natural soil. At the pumping of such products it is thus important to use such a low speed pump as possible which can accept and transport large particles as well as to provide the highly viscous product ample time to flow up to the pump.
Today, so called modified Archimedes screw pumps are used having a multiple-toothed radially positioned sealing and pressure build-up disc, which is in engagement with the screw. An example of such a screw pump is disclosed in SE-B-8101863. Because of the geometry of the sealing disc the meshing part of the screw can not be machined in a simple way, which implies bad sealing and high wear on the included parts. The displacement is limited by screw flanks during one screw turn and the cylinder wall and two successive teeth on the sealing disc. In practise the whole screw diameter cannot be used since the screw must have a supporting centre shaft, which steals a part of the displacement of the pump. Moreover the relatively large sealing disc demands large space which results in that the pump becomes bulky and cannot by made compact enough with regard to capacity for fitting into xe2x80x9cbutter-worthxe2x80x9d-spaces.
It further known from EP-A-0 523 113 a screw pump of the type given in the preamble where the screw area of the screw, including thread grooves and thread walls, are formed by a generatrix which is a part of a circle arc, the circle of which is rotatable around an eccentric rotation axis when the generatrix rotates around the axis of the screw and is displaced along this with a velocity being proportional to the rotational velocity of the screw and wherein the sealing device comprises at least one circular disc the eccentric rotational axis of which, being arranged transverse to the axis of the screw coincides with the rotation axis of the generatrix. i.e., the rotation axis of the disc is a tangent to the generatrix.
This screw pump in a commercial embodiment, has at a pumping volume of 65 m3/hr a length of 0.88 m and a largest diameter of 0.44 m. This pump has a screw being mounted in both ends and which comprises in total three thread turns. The commercial product is not capable of inter alia due to is mounting at its xe2x80x9clowerxe2x80x9d end to come close enough to a bottom surface and is thus not capable of sucking clean enough at a total emptying of a tank. Further, the pump can not work in the collecting spaces present.
The present object is now better understood with regard to what has been cited above and this object in accordance with above shall be completed with obtaining a pump which can work so close as possible to its suction surface in order to facilitate a maximum elimination of the pumped product. Simultaneously as the pump has this ability it shall still accept large particles in relation to its diameter.
It has now surprisingly been shown possible to be able to solve the present problem and fulfill the object by means of the present invention which is characterized in that the screw is mounted in one end, and that the screw has a radially, partially extending broad flange without any rise in a cross-section immediately above the sealing device.
Further characteristics are evident from the accompanying claims.
By means of the present invention a very compact screw pump is achieved having with regard to its displacement a high capacity. A screw pump of the diameter 200 mn has a capacity of 70 ton of oil/br, when the effective screw length is less than 200 mm and at a diameter of 141 mm an effect of about 25 ton/hr is achieved.
By means of the present invention, further, a deepening of the screw area in axial direction is obtained in combination with the broad flange, into the screw body, whereby a thickness of the screw parts is obtained, with the exception of the central longitudinal axis part, which material thickness is essentially the same all over the screw.
Further, there is achieved using a preferred embodiment of the present invention, an overlap of the working Areas of the sealing discs amounting to at least 10xc2x0, whereby any internal leakage in the screw pump is markedly reduced.
By increasing the size of the diameter in the inlet part of the screw pump a markedly better feeding of the pumped material to the screw pump is obtained. This increase of size is obtained as an effect of the improved overlapping of the working areas of the sealing discs which in turn is obtained by an enlargement of the arc of the abutment lines of the sealing discs.